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04' MPG dropped to 43, short commute, 90F

Discussion in 'Gen 2 Prius Fuel Economy' started by AllenZ, Jul 14, 2011.

  1. AllenZ

    AllenZ Active Member

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    Guys,

    I used to get 55-58 MPG when my commute was 65 miles one way, 85% freeway. But these days I noticed that when AC runs hard and trips typically 16 miles, 40 MPH, mpg dropped to 42-45. That worried me a little bit.

    Driving style: conservative. I start off normally, but I use N gear as soon as passing speed limit. (this factor alone should enhance mpg by 10 I guess. I used this technique on my 98' Lexus LS400, 4 Ltr V8 engine, once got 38 mpg on freeway, while the sticker says 18/25). This is the main reason I worry that something is wrong with my prius.

    Typical range: 16 miles, all local.
    Tire pressure: 40 psi
    AC runs hard, outside temp 90 degrees, I set temp to 72 inside. (I guess this is the main reason for mpg drop, but I don't think it should drop that much)
    I recently changed oil and trans oil.
    My battery is a little bit weak, but no warning light. If I forget to turn off head lights, it needs jump in 10 minutes.
    Mileage is 143K, 2004 model.
    Water pump changed by Toyota free of charge.

    Any clue? or it sounds normal and I don't need to worry about it?
    Thanks!
     
    #1 AllenZ, Jul 14, 2011
  2. seilerts

    seilerts Battery Curmudgeon

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    If the 12V is original, replace it.
     
    #2 seilerts, Jul 14, 2011
  3. AllenZ

    AllenZ Active Member

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    Sorry, the mileage is 142K. I edited it in OP.
    I bought the car at 110K miles, not sure if that's the original battery. But my understanding is that the 12V does not have anything to do with mpg, since it only power up the system, and maybe runs the sound system and some lights. The hybrid battery suppose to be long lasting, should not show deterioration at this mileage. Am I correct on this?
     
    #3 AllenZ, Jul 14, 2011
  4. wb9tyj

    wb9tyj 2017 Prius Prime Advanced

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    If your 12v small battery is low...your mileage will be reduced...due to the fact the system tries to charge a "weak" battery, which in turn runs the gas engine more to maintain the charging...hence a drop in gas mileage....i have 2 prii an 04 and 09...my 04 i have had 3 12v batteries changed over 150000 miles...so i would change the 12v ASAP before you hit the start button and nothing boots up...
     
    #4 wb9tyj, Jul 14, 2011
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  5. F8L

    F8L Protecting Habitat & AG Lands

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    If you have had to jump your battery even a few times then I would suggest checking its capacity and possibly replacing it. The battery is fairly fragile and after that many miles and a few jump starts it is probably on its last leg.
     
    #5 F8L, Jul 14, 2011
  6. AllenZ

    AllenZ Active Member

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    :cool:
    That seems make sense. Thanks!
     
    #6 AllenZ, Jul 15, 2011
  7. Danny Hamilton

    Danny Hamilton Active Member

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    It doesn't sound like anything to worry about. It is probably time to replace the 12V battery anyhow, but I suspect a large part of the MPG change is due to your change in driving habits. Slower speeds will increase MPG a bit, however a reduction in average trip length from 65 miles to 16 miles will certainly reduce MPG. Add to that an increase in A/C usage, and then I'm guessing that at 40 mph you are no longer 85% on the freeway so perhaps you are encountering more stop-and-go traffic (stop signs, traffic lights, intersections, etc). These both will also reduce MPG.
     
    #7 Danny Hamilton, Jul 21, 2011
  8. cyclopathic

    cyclopathic Senior Member

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    put it air on recirculate, fan on 40% and increase temperature to 80-82. That will improve you MPG

    with respect to 12v battery the worn one is known to cause many issues, loss of MPG, etc. Test it under load; with hybrid off, turn lights and check voltage. If it is below 11.5, replace ASAP good luck
     
    #8 cyclopathic, Jul 22, 2011
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